1. Field of the Invention
Aspects of the present invention relate to a printing medium detecting device, an image forming apparatus having the printing medium detecting device, and a method to detect a printing medium.
2. Description of the Related Art
An image forming apparatus, such as a copier, a printer, a facsimile, and a multi-functional device, includes an image forming unit to form an image on a printing medium supplied from a paper cassette, and a printing medium detecting device to detect an amount of printing media remaining in the paper cassette. FIGS. 1A and 1B show examples of a printing medium detecting device 1 of an image forming apparatus.
As shown in FIGS. 1A and 1B, a printing medium detecting device 1 of a general image forming apparatus includes a knock up plate 2a to support printing media P housed in a paper cassette 2, and a display lever 3 to rotate about a hinge axle 3a. 
According to the construction illustrated in FIGS. 1A and 1B, the display lever 3 rotates about the hinge axle 3a while linking to the knock up plate 2a which moves in a vertical direction according to the weight of the stacked printing media P. Accordingly, a user may check a number of sheets of the printing media P remaining in the paper cassette 2 by observing a location of the display lever 3.
However, in this printing medium detecting device 1, the user checks the display lever 3 directly with his or her naked eye. Furthermore, when multiple users are using a single image forming apparatus, such as a network printer, multiple image forming signals may be simultaneously input. Accordingly, in this situation, it is not easy to predict whether the amount of printing media P stacked in the paper cassette 2 will be sufficient when several image forming operations are simultaneously performed.
In other words, in situations where multiple image forming operations are performed and the printing media P housed in the paper cassette 2 is not sufficient to perform each of the image forming operations, users do not know whether the number of sheets of the printing media P is sufficient to perform each of the image forming operations until after the image forming commands have been input.
Therefore, problems arise, including that the operation time required for the image forming operations becomes longer if the printing media P becomes depleted in the middle of the printing operations, resulting in a decrease in the operation efficiency.